A heat engine is an energy system that performs the conversion of thermal energy from a heat source or heat reservoir to mechanical work. A variety of heat sources may be employed to power a heat engine. These heat sources may include, but not limited to, combustion gases from a combustion chamber, exhaust gases from a diesel engine, gasoline engine, or gas turbine engine, flue gases and hot fluids from industrial furnaces or processes, and various non-combustion heat sources related to solar, geothermal, nuclear, or biological energy.
Unlike a heat engine that employs a working fluid without the change of chemical compositions, an internal combustion engine, strictly speaking, does not work in cycles due to the change in chemical composition of the working fluid. Traditionally, however, the operation of an international combustion engine is simplified as cycles for the convenience of analysis. In this regard, the chemical energy associated with a fuel is converted into thermal energy through combustion, and the thermal energy released during combustion is absorbed by the compressed working fluid over a certain time period in a cycle. This time period may be measured in terms of crank angle (CA) or shaft rotating angle. As a result, the thermal energy released in the combustion and absorbed by the working fluid may be expressed by the following relation:
                              Q          c                =                                            ∫                              θ                s                                            θ                e                                      ⁢                                                                                Q                    .                                    c                                ⁡                                  (                  θ                  )                                            ⁢                              ⅆ                θ                                              =                                                                      Q                                      .                    _                                                  c                            ⁡                              (                                                      θ                    e                                    -                                      θ                    s                                                  )                                      =                                                            Q                                      .                    _                                                  c                            ⁢              Δ              ⁢                                                          ⁢                              θ                c                                                                        (        1        )            where {dot over (Q)}c is the instantaneous combustion heat release rate and {dot over ( Q)}c is the average heat release rate over the combustion duration, both having a unit of J/CA degree, θs is the crank angle at which combustion starts in the combustion chamber, θe is the crank angle at which the combustion ends, and Δθc is combustion duration in CA degree. Due to the explosive nature of combustion in a combustion chamber and high average heat release rate, {dot over ( Q)}c, the combustion duration is normally small, on the order of 30-40° CA.
For a heat engine, in which the compressed working fluid extracts heat from an external heat source, such as a combustion gas from an external combustion chamber, exhaust gas from a diesel-type, gasoline-type, or gas-turbine engine, solar energy source, geothermal heat source, nuclear energy source, or biological energy source, the acquisition of the thermal energy by the working fluid is normally through a heat exchanger that facilitates heat transfer from the heat source to the working fluid driven by a temperature difference between the heat source and the working fluid. This heat transfer would occur during a time period in a cycle, which could be also measured through crank angle (or shaft rotating angle):
                              Q          HT                =                                            ∫                              θ                1                                            θ                2                                      ⁢                                                                                Q                    .                                    HT                                ⁡                                  (                  θ                  )                                            ⁢                              ⅆ                θ                                              =                                                                      Q                                      .                    _                                                  HT                            ⁡                              (                                                      ϑ                    2                                    -                                      θ                    1                                                  )                                      =                                                            Q                                      .                    _                                                  HT                            ⁢              Δ              ⁢                                                          ⁢                              θ                HT                                                                        (        2        )            
where {dot over (Q)}HT is the instantaneous heat transfer rate and {dot over ( Q)}HT is the average heat transfer rate over the heat transfer duration, both having a unit of J/CA degree, θ1 is the crank angle at which the heat transfer begins, θ2 is the crank angle at which the heat transfer ends, and ΔθHT is the heat transfer duration in CA degree.
It is well known that the heat absorbed by the working fluid in a cycle, either Qc or QHT, may predominantly determine the power output of an engine at a given engine speed. In many applications, however, with a comparable size the average heat transfer rate, {dot over ( Q)}HT, in a heat engine may be at least an order of magnitude lower than the average heat release rate of an internal combustion engine, {dot over (Q)}c. As a result, the duration of heat transfer, ΔθHT, must be significantly longer than the heat release duration, Δθc, of an internal combustion engine to provide a sufficiently high QHT for building a heat engine that could be practically viable. It is also well known that for an engine operating at a given speed, both power output and thermal efficiency will depend on the number of strokes per power stroke in a cycle. For a given heat input in a cycle and a given operating speed, a smaller number of strokes per power stroke will have the benefits of increasing both power output and thermal efficiency due to reduced frictional losses. Therefore, it is very important that an increase in the duration of heat transfer not result in an increase in the number of strokes per power stroke in the cycle.
Turning now to a rotary combustion engine such as a Wankel rotary combustion engine, one of the difficulties is the cooling of rotor seals. In a piston engine, the piston may be adequately cooled by engine oil from an oil sump and by heat conduction from the piston surface to the cylinder wall due to a large contact surface between the piston and cylinder wall. However, these cooling means may be substantially eliminated due to a different structure of the rotary engine. As a result, the rotor temperature, particularly the temperature of the seals at the apexes of the rotor, may be substantially higher than that of a piston. Additionally in a rotary engine, the engine housing is constantly heated on the side of combustion chamber and cooled on the side of intake port, leading to a large temperature gradient along the circumference of the housing, which may cause uneven thermal expansion.